The present invention relates to an absolute encoder, and more particularly relates to an encoder capable of detecting the absolute displacement of a mobile body from a reference position.
An incremental rotary encoder is one type of conventional encoder. Such an incremental rotary encoder includes a pitch signal track, a pair of magnetic sensors, a divider circuit connected to the magnetic sensors and a pole calculator circuit connected to the magnetic sensors in parallel to the divider circuit.
The pitch signal track is formed along a prescribed circular path on a magnetic recording medium, which is notatably in association with a rotary body. Magnetic information in the form of a series of continuous sine waves of a constant wave length .lambda. is stored on the pitch signal track. Each of the magnetic sensors is in the form of a magnetic resistor element formed on a glass substrate. The magnetic resistor element changes its inherent resistance in response to the intensity of the magnetic field in which it is located. This change in resistance is utilized for reading of the sine wave magnetic information stored in the pitch signal track. Facing the pitch signal track, the pair of magnetic sensors are spaced apart from each other by a distance equal to (k.+-.1/4).lambda., k being an integer. The pitch signal track is movable in one longitudinal direction with respect to the magnetic sensors. The magnetic resistor element of each magnetic sensor changes its resistance depending on the intensity of magnetic field applied by the pitch signal track to produce a signal having a level corresponding to the relationship in position between the pitch signal track and the magnetic sensors. When one period of the sine wave magnetic information on the pitch signal track .theta. is in a range from 0 to 2.pi., the magnetic sensors produce level signals sin .theta. (A phase) and cos .theta. (B phase), respectively. As the pitch signal track moves with respect to the magnetic sensors, the magnetic sensors produce a pair of detection signals sin .theta. and cos .theta. with a phase difference of .pi./2.
A divider circuit is also provided which includes A/D converters connected to the magnetic sensors and an angle calculator connected to the A/D converters. The A phase detection signal sin .theta. is converted by one A/D converter to produce digital data Da whereas the B phase detection signal cos .theta. is converted by the other A/D converter to produce digital data Db. At the angle calculator, an angle data .theta. is calculated from the two data Da and Db. This angle data .theta. indicates the position of either of the magnetic sensors within one magnetic domain on the pitch signal track.
There is also provided a pole calculator circuit which includes three wave shaping circuits and a direction discriminator circuit. Two of the wave shaping circuits are connected to the magnetic sensors for wave shaping of the A phase and B phase detection signals. The first and second output signals of these wave shaping circuits take the form of square waves which are phased from each other by .pi./2. When the magnetic recording medium rotates in the positive direction the first output signal precedes whereas, when the magnetic recording medium rotates in the negative direction, the second output signal precedes. The direction discriminator circuit determines the direction of the rotation depending on, for example, the level of the second output signal at rising of the first output signal. The output signal of the direction discriminator circuit is supplied to the up-down shift terminal of a counter. In accordance with the counting mode defined by the output signal from the direction discriminator cirucit, the counter counts the first output signal from the first wave shaping circuit. In one example, the counter operates in the up-count mode when the magnetic recording medium rotates in the positive direction. A zero point sensor is arranged facing the pitch signal track so that it should issue a zero point signal at every passage of the zero point on the pitch signal track. This zero point signal is converted into a zero point pulse after passage through the third wave shaping circuit of the pole calculator circuit. The zero point pulse is then passed to the reset terminal of the above-described counter. As a result, the counter is reset every time the zero point sensor passes by the zero point on the pitch signal track. Consequently, the count value N of the counter corresponds to the number of the magnetic domains on the pitch signal track passed by the magnetic sensors within a region between the current positions of the magnetic sensors and the zero point position. A combination of this count value N with the angle of data .theta. put out by the angle calculator is issued as an absolute position data Dout.
Having the above-described construction, the incremental rotary encoder suffers from several drawbacks. Even when the zero point on the pitch signal track arrives at the positions of the magnetic sensors, the counter in the pole calculator circuit cannot be initialized until the zero point signal is detected. As a result, the absolute position data Dout is inaccurate during this period. Data stored in the counter is destroyed when supply of power is accidentally interrupted. In order to avoid such undesirable data loss, a backup power supply has to be reserved.
An optical, absolute rotary encoder is another type of conventional encoder used for detecting an absolute displacement. This type of encoder includes, as an inforamtion recording medium, a circular disc secured to a center shaft which is mechanically coupled to a rotary body for synchronized rotation. A plurality of annular tracks are concentrically defined on this circular disc for storage of an angle information which indicates a rotation angle of the circular disc from a given reference position. For example, four sets of annular tracks are defined so that the first to fourth tracks should correspond to the first to fourth bits of the angle information. Each annular track is provided with several arcuate slits spaced from each other so that each open section should indicate a bit value "1" and each blind section should indicate a bit value "0". Utilizing the angle information given by the four sets of annular tracks, rotation of the circular disc from the reference position is detected at an accuracy of 22.5.degree..
A light source is arranged facing the plane of the circular disc in order to emit light beams towards the annular tracks on the disc. Each light beam passes through the disc only when it is emitted toward the open sections, i.e. the slits, in each annular track. Four sets of photodiodes are arranged on the side of the circular disc opposite to the light source in order to receive light beams which traveled via the open sections in the disc. The anode of each photodiode is grounded whereas the cathode is connected to a given power source via a resistor. The number of the photodiodes depends on the number of the annular tracks on the circular disc. The cathode of each photodiode is also connected to one input terminal (inverting) of a comparator. The other input terminal (non-inverting) of the comparator is connected to the slider of a variable resistor. One end of this resistor is grounded whereas the other end of this resistor is connected to a given power source. By changing the position of the slide in each variable resistor, the non-inversion level, i.e. the discrimination level, of the associated comparator is adjusted.
As the rotary body rotates, the circular disc rotates also in synchronism. Each light beam emitted by the light source goes through the circular disc only when it is directed to any one of the open sections formed by the slits in the annular tracks. On receipt of such a light beam, a corresponding photodiode is rendered conductive and the electric potential of its cathode falls. When no light beam is received, a corresponding photodiode remains non-conductive and the electric potential of its cathode remains high. The electric potential of each cathode is discriminated by an associated comparator for issue of an angle data. The above described rotary encoder produces angle data representative of the rotation angle of its associated rotary body.
The above-described absolute rotary encoder also differs from several drawbacks. In order to enhance the degree of resolution, the absolute position data must be provided with a comparatively larger number of bits and such large number of the bits inevitably involves as corresponding increase in the number of necessary annular tracks and of the sensors, i.e. the photodiodes.